AC motors are used in a variety of applications, including vehicle applications, and AC induction motors are desirable for having a simple, rugged construction, easy maintenance, and cost-effective pricing. The AC motors used in vehicle applications are typically controlled via a voltage source inverter such that the motor phase currents are sinusoidal. Supplying a sinusoidally shaped input current to the AC motor typically produces torque without additional harmonics which can be a source of torque pulsations in the AC motors.
In vehicle applications, one design consideration is to maximize the utilization of the available DC bus voltage (e.g., provided by a battery). Maximization of the bus voltage utilization generally improves the high speed power and overall system efficiency. Some AC motors are permanent magnet (PM) machines. PM machines typically have high power density and high efficiency characteristics and are thus well-suited for vehicle propulsion applications. Electric machines have a current limitation, due to the current limits of the voltage source inverter, and a voltage limitation, due to the available DC bus voltage. At higher speeds, the PM machine, without voltage control, produces a machine flux, or a back EMF, that may increase beyond the DC bus voltage. For example, the phase voltage of a PM machine increases as the speed of the machine is increased. Above a predetermined speed, the phase voltage of the PM machine becomes greater than the bus voltage. To retain current control of the PM machine, the back EMF is reduced using field-weakening. In PM machines, the magnet flux cannot be inherently reduced, thus a demagnetizing current is typically applied to reduce the magnet or total flux of the PM machine.
To implement an efficiency-optimized control of the PM machine, the non-linear characteristics of the PM machine may be measured and used to develop a non-linear machine model. This model is used to determine efficiency-optimized control parameters, and these control parameters are typically added to the counter as look-up tables for efficiency-optimized control of the machine. The control parameters may also be determined within the voltage and current limits. During an ideal operation, a feed-forward control using these control parameters is generally sufficient to provide stable control of the PM machine under steady state conditions. To retain current control at high speeds, when the available voltage is limited, additional assistance may be needed especially during transient operations or in the event of a mismatch between the actual machine parameters and the measured parameters. A field weakening voltage loop is typically used to correct the errors between the model and the actual machine parameters for a stable machine operation.
Some strong magnet flux PM machines have a high no-load loss (e.g., a spin loss) and fault problems. A weak flux PM machine may be selected where the magnet flux is purposely kept low to avoid the problems associated the PM machines. Currently, d-axis current control techniques have been used to field weaken the back EMF in strong magnet flux PM machines. For example, a negative d-axis current may be applied to produce a demagnetizing flux component that reduces the magnet flux and the magnet back EMF. These d-axis current control techniques have limited success with weak flux PM machines due to the weak influence of the d-axis current on the machine voltage. For example, in the non-linear overmodulation region of operation, the weaker influence of the d-axis current on the machine flux may impair field weakened operation of the weak flux PM machine by increasing the total voltage magnitude instead of decreasing the same. D-axis current control can also fail in a strong flux machine that operates at high speed with a large demagnetizing current. Under a large demagnetizing current, d-axis flux may reverse sign (i.e., become negative) for a strong flux machine. The reversal of d-axis flux occurs in a weak flux machine at a much lower demagnetizing current and hence at a lower speed. As previously mentioned, the d-axis current has a weak influence on voltage in a weak flux machine. Moreover, due to the reversal of the sign of the d-axis flux, which may also be true for a strong flux machine, an attempt to lower voltage by applying more d-axis current generally increases the machine terminal voltage, thus destabilizing the demagnetizing control.
By overcoming machine voltage, current can be produced in the machine. In addition to the back EMF, the current regulator should overcome the resistive drop and the inductive drop. The inductive drop can be high, especially in the q-axis for a machine with reluctance (e.g., an interior PM machine or synchronous reluctance machine). By lowering machine terminal voltage, current control can be retained. For some high flux PM machines, lowering the PM back EMF at high speed by injecting demagnetizing current (e.g., negative d-axis current) indirectly lowers the machine terminal voltage due to a strong influence (e.g., PM field) on the voltage. However, for other machines, such as weak flux PM machines, synchronous reluctance machines, or even for some strong PM flux machines, controlling the d-axis current to reduce back EMF may not have a desirable effect on the machine terminal voltage.
Accordingly, it is desirable to provide a method for controlling permanent magnet or synchronous reluctance motor drive systems that reduces the machine terminal voltage while retaining current control particularly at high speeds. Additionally, it is desirable to provide a control system for PM or synchronous reluctance motor drive systems that reduces the machine terminal voltage while retaining current control particularly at high speeds. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.